With the recent tendency of Japanese eating habits toward the Western style, the demand for meat has been increasing. Therefore, meat manufactures need to develop an effective feeding method. On the other hand, the Japanese livestock industry should further improve productivity to compete on the world market.
Examples of methods for improving productivity in the pig industry include enlargement of the litter size, elevation of the rate of raising infants, and increment of the farrowing frequency. Although the litter size and the rate of raising newborns have improved by progress in livestock breeding and by improvements of feeding management facility and environmental sanitation, these effects are still insufficient.
On the other hand, improvement of the conception rate or prevention of breeding maladies is another important subject for the improvement of productivity. In order to breed pigs, it is necessary to repeat the cycle of estrus, mating, conception and farrowing efficiently. However, the generation of breeding maladies has been increasing in recent years because, for example, of the difficulty in closely observing each pig for feeding management due to the expanding management scale of pig farms, thus causing significant managerial loss.
In the case of pigs, a decrease in their breeding activities in the hot season is a problem which should be resolved. Its boar-side causes include the onset of diseases such as orchitis and the like, a lack and decrease in copulative passion, the deterioration of semen conditions and the like in the hot season. Especially, it is substantially difficult to check the deterioration of semen conditions in the case of natural mating, for example, so that it is mostly past hope when a breeding malady in boars is found. In addition, since semen conditions in the hot season are unstable, preservable days of semen are also reduced by half, hence posing another problem from the viewpoint of artificial fertilization.
In consequence, in order to effect an improvement of the breeding activity or the prevention of breeding maladies, various means have been attempted such as the administration of antibiotics and hormones. However, such means are not always sufficient enough because of additional problems in terms of side effects or safety of the feed. Because of this, great concern has been directed toward the development of a means by which breeding activity can be improved efficiently with less labor and cost, particularly a means which renders possible stable conception and provision of spermatozoa throughout the year by preventing a decrease in the fertilization capacity of boars in the hot season.